The present invention relates generally to a method of perforating the casing in a well from which oil and/or gas can be produced.
In order to produce hydrocarbon fluids from subterranean formations, a borehole is drilled from the surface down into the desired formations. Typically, cylindrical casing is placed in the borehole, thereby defining a hollow wellbore. In order for the hydrocarbon fluids to flow from the surrounding formations into the wellbore and up to the surface, it is necessary to perforate the casing. This is typically done using a perforating gun, a downhole tool that detonates explosive charges at selected locations in order to form holes in the casing.
Because the fluids in the formation are under pressure, a choice must be made whether to perforate the well with the bottom-hole pressure in the wellbore lower or higher than the formation pressure. The former condition is referred to as “underbalanced” and the latter condition is referred to as “overbalanced”.
It has been common in recent years to perforate the casing of a well in an underbalanced condition. For high pressure reservoirs, an underbalanced condition is easy to achieve. However, if the pressure in the formation is relatively low, it may be difficult or impossible to achieve a lower pressure in the wellbore, especially considering the great depth of some wells. In that situation, the well operator does the best that is possible under the circumstances, and perforates the well with whatever liquid is in the wellbore.
Experience has shown that different wellbore fluids can have significant effects on production rates. For example, perforating with brine in the wellbore usually results in two-phase (oil and water) flow in the formation pores. This condition is known to deleteriously affect production rates. It is also possible to perforate with an oil-based mud in the wellbore. This eliminates the two-phase flow in the formation pores, but can results in plugging some pores with the solid (clay) portion of the mud.
There is a need for improved methods of perforating well casing that reduce or eliminate at least some of the above-described problems.